The present invention relates generally to laser-trimming and more specifically to laser-trimming above a semiconductor material layer which is on a dielectric material layer.
Laser-trimming of thin-film resistors is used extensively to produce improved accuracy in analog integrated circuit technology. In integrating laser trimming into dielectrically isolated circuit technology, a difficulty peculiar to dielectric isolation has been identified. Trimming is generally accomplished by use of an infrared laser for improved control. Silicon is nearly transparent at the wavelengths used; this results in laser energy penetrating to the bottom of the dielectrically isolated island, reflecting back and transferring some of the reflected energy back to the resistor. The result is poor control due to interference effects. These effects are variable due to changes in dielectrically isolated island-depth and proper control over trim-energy hence becomes very difficult.
An existing technique addresses the problem by simply placing the thin-film resistor over the polycrystalline silicon used to support the dielectrically isolation regions. The polysilicon is much thicker than the single-crystal islands (typically 10 mils vs. 1 mil) and energy scattering off the grain boundaries soon dissipates the laser beam. The resulting lack of reflection and interference produces enhanced controllability. The resistor is deposited over a polycrystalline surface which is not perfectly flat but in fact possesses considerable relief at the grain boundaries. In addition, large polycrystalline areas tend to "dish out" during the grind and polish operation which complicates laser-focusing and reduces photoresist definition, resulting in poor control over resistor geometries.
One solution is suggested in U.S. Pats. Nos. 4,468,414 and 4,510,518 to N. W. Van Vonno. An opening is provided in the dielectric isolation at the bottom of the single crystal island exposing the polycrystalline support. Thus, infrared radiation passes into the support and does not reflect back to the top surface of the single crystal island.
In some applications, all islands must be dielectrically isolated, and thus the solution of the aforementioned Van Vonno patent cannot be used. In such applications, Van Vonno, et al in U.S. Pat. No. 4,594,265, roughens the surface at the bottom of the single crystalline island such that the surface of the dielectric isolation is substantially non-reflective.
Both of these methods provide solutions to the reflected back energy but require additional processing steps to achieve the results.
Although this technology has been generally directed to dielectrically isolated islands, other semiconductors with insulator interfaces are also being used. For example, silicon-on-insulator (SOI) are becoming very popular in design. They would experience the same problem of laser trimming since the silicon would be transparent to the laser with the supporting insulator structure forming a reflective surface.
Thus it is an object of the present invention to provide a method of fabricating laser trimmed material over semiconductor regions which are backed by an insulative layer which has reduced reflectivity.
A still further object of the present invention is to provide a method of laser trimming material on dielectrically isolated integrated circuit regions without additional processing steps.
An even further object of the present invention is to provide an integrated circuit on which laser trimmable material is to be applied and trimmed which includes a semiconductor region backed by a dielectric layer.
These and other objects are attained by selecting the thickness of the dielectric layer to be approximately an integer multiple of one-half of the wavelength of the laser light in the dielectric layer. By defining the relationship between the laser light in the dielectric isolation layer and its thickness, the dielectric isolation layer becomes invisible to the light and does not produce the undesirable reflection. The wavelength of the laser light in the isolation layer is equal to the wavelength of the laser light divided by the refractive index of the insulated layer. Thickness may be in the range of an interger multiple of one-half of the wavelength of the laser light in the isolation plus or minus one-eighth of the wavelength of the laser light in the dielectric isolation. The dielectric isolation may be in an integrated circuit having semiconductor islands in dielectric tubs or may be a layer of silicon on a layer of dielectric isolation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.